Which to keep! Pick one of these two soeakers

The sonics you are describing have a lot to do with your equipment.

Happy Listening.

If you only have the space for 1 keep the Spendors.  They are a bit more versatile, and should sound better on most songs.  However with the right song, panels can do magic no box speaker will match. I'd do my best to keep both since they are very different speakers.  I've got a box/horn (Dynaudio/Klipsch) pairing myself.  One in the bedroom and the other in the living room. 

Just my 2 cents.. if financially viable keep both. You have two very different designs , with different strengths and weaknesses. Keep them in your main listening space, swap them, mix and match to musical performance strengths. Disclaimer : I own way too many full range speakers because of this approach. 
 

I have two glasses of Red Wine in front of me.  Can someone tell me which one I prefer the taste of?  Sorry, but if you have two sets of speakers in your room, and you can't tell which one you prefer, I don't understand how the opinions of others are going to help?  If you really can't pick one based on your own listening experience, then I would say sell the one that will bring the most money.  

Spendor S100 in a landslide great classic speakers.